Calibrated microelectromechanical microphone

ABSTRACT

A MEMS microphone comprising a MEMS transducer having a back plate and a diaphragm as well as controllable bias voltage generator providing a DC bias voltage between the back plate and the diaphragm. The microphone also has an amplifier with a controllable gain, and a memory for storing information for determining a bias voltage to be provided by the bias voltage generator and the gain of the amplifier.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/847,319, filed Sep. 26, 2006, entitled “Calibrated Microphone”, whichis hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to calibrated microphones and inparticular microelectromechanical microphones comprising a memory havingcalibration data which are used for setting electrical parameters of themicrophone.

BACKGROUND OF THE INVENTION

Microelectromechanical (“MEMS”) microphones are currently supplied witha fixed DC bias voltage between the diaphragm and backplate structuresduring normal operation. Under microphone fault conditions in connectionwith a so-called diaphragm collapse, a certain manipulation of the DCbias voltage to remove or decrease attractive electrostatic forcesbetween the diaphragm and backplate has been proposed and published inEP 1 599 067 A2.

US 2006/062406 A1 discloses a condenser microphone comprising aprogrammable DC bias voltage for a microphone condenser transducer and amemory for storing a set value of the DC bias voltage. WO 01/78446 A1discloses an electret microphone comprising a variablesensitivity/variable gain circuit coupled between the electrettransducer and a buffer amplifier.

Other references related to calibrated microphone systems and methodsare: U.S. Pat. No. 4,631,749, U.S. Pat. No. 5,051,799, U.S. Pat. No.5,029,215, US 2003/0198354 A1, and US 2005/0175190 A1.

A significant problem in producing MEMS condenser microphones with highyield is that the compliance or tension of the MEMS microphone diaphragmvaries according to a number of manufacturing parameters that aredifficult to accurately control. The absolute values of physical ormechanical parameters from silicon wafers (e.g. mechanical stiffness,electric resistance, transistor trans-conductance) may easily vary by+/−20% or more. This is a significant disadvantage for well-controlledMEMS microphone fabrication.

Other physical parameters of a MEMS microphone also vary, e.g. diaphragmarea, air gap height, i.e. the distance between the diaphragm and theback plate. Compared to a standard “macroscopic” microphone, in whichthe air gap height is larger than 30 or 50 μm, the air gap height inMEMS transducers is typically 5-10 μm or even smaller. The smalldimensions of MEMS microphones impose severe limitations on how a DCbias voltage can be adjusted to compensate for a non-nominal acousticsensitivity. Adjusting, the DC bias voltage to a high value may causethe collapse threshold, in terms of dB SPL, to move to an unacceptablelow value.

The influence of varying parameters of electrical components encounteredin the manufacture process of integrated semiconductor circuits, such asCMOS circuits, is usually less significant to the performance anduniformity of MEMS microphones. However, a certain influence onperformance parameters such as amplifier gain and impedance remains.This influence is particularly difficult to eliminate in high volume andlow-cost MEMS microphones where low-complexity amplifier topologies areessential to keep die area, and thereby cost, low. Consequently, itwould be advantageous to compensate for these performance parametervariations.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to a MEMS microphone assemblyincluding a microphone housing. The MEMS microphone assembly comprises asound inlet, a MEMS transducer element, a controllable bias voltagegenerator, a memory, a controllable amplifier, and a processor. The MEMStransducer element has a back plate and a diaphragm displaceable inrelation to the back plate. The controllable bias voltage generator isadapted to provide a DC bias voltage between the diaphragm and the backplate. The memory device is for storing information, including amplifiergain setting information. The controllable amplifier receives anelectrical signal from the MEMS transducer element and provides anoutput signal. The controllable amplifier is adapted to amplify theelectrical signal from the MEMS transducer in accordance with anamplifier gain setting. The processor is adapted to retrieve theinformation from the memory. The processor controls the gain of theamplifier in accordance with the amplifier gain setting information fromthe memory, and controls the bias voltage generator to provide a DC biasvoltage in accordance with the information from the memory.

In the present context, a MEMS-based transducer is a transducer elementwholly or at least partly manufactured by application of MicroMechanical System Technology. The miniature transducer element maycomprise a semiconductor material such as Silicon or Gallium Arsenide incombination with conductive and/or isolating materials such as siliconnitride, polycrystalline silicon, silicon oxide and glass. Alternativelythe transducer element may comprise solely conductive materials such asaluminium, copper, etc., optionally in combination with isolatingmaterials like glass and/or silicon oxide. Preferably, a MEMS microphoneassembly in accordance with the present invention is a small orsub-miniature component such as one having an extension, in the plane ofthe diaphragm, of less than 7.0×5.0 mm or less than 5.0 mm×4.0 mm, suchas 3.5 mm×3.5 mm, or even more preferably less than 3.0 mm×3.0 mm. Thesedimensions are suitable for the integration of the MEMS microphoneassembly into a wide range of portable communication devices such asmobile terminals, mobile phones, hearing instruments, head sets, activenoise protection devices etc.

According to the invention, a combination of DC bias voltage adjustmentand gain adjustment allows the provision of MEMS microphone assemblieswith a well-defined collapse threshold as well as retaining a desiredpredetermined or nominal acoustic sensitivity.

Preferably, the MEMS transducer element has a distance, normally calledthe air gap height, from the back plate to the diaphragm (in anon-biased state) of 1-10 μm, such as 2-5 μm. In addition, acontrollable bias voltage generator for a MEMS transducer normally isadapted to generate a DC bias voltage in the interval of 5-20 V.

The present memory may comprise memory circuitry of any type, such asRAM, PROM, EPROM, EEPROM, flash, and is normally non-volatile.Particularly interesting memory types are one-time-programmablememories, such as memories based on fuse-link technology. Preferably,such memories are programmable while mounted in the microphone assembly.

The amplifier may comprise a microphone pre-amplifier operativelycoupled to the MEMS transducer element. Preferably, the gain isadjustable by altering electrical parameters of circuit components likeresistors and capacitors, such as components of a feed back circuit,coupled to the amplifier. Amplifiers may be merely a single transistoramplifier or buffer, preferably based on a CMOS transistor, or may bemore complex circuits such as multistage operational amplifiers.

The DC bias voltage generator preferably is a circuit type which isadapted to provide an essentially fixed DC voltage by voltage divisionor voltage multiplication or voltage regulation. A set-up as simple as abattery supply line and an adjustable voltage divider may be used, orthe power feeding means for the amplifier may be used with a suitablevoltage regulator. A preferred voltage multiplier embodiment comprisesthe well-known Dickson charge pump.

In one embodiment, the processor comprises the amplifier, the processorproviding the output signal in accordance with the information from thememory. Thus, the same processor handles both operations.

Another advantage is the more compact set-up which makes it even easierto fit all elements into a single package, such as by using chip-scalepackaging, which makes the present highly adjustable microphone with apotentially high yield extremely compact.

The programming of the DC bias-voltage may be based on a measurement ofthe characteristics of the actual MEMS transducer element or a sampleMEMS transducer element, or collection of samples, placed on the samewafer as the actual MEMS transducer element.

As the MEMS transducer normally is made in batches on semiconductorwafers, methods are known for estimating parameters of all elements of asingle wafer or all elements of the batch of wafers that may comprise aplurality of individual wafer such as 10-48 individual wafers. The airgap height as well as the compliance/stiffness of the diaphragm, etc,may be measured or estimated in this manner, whereby a suitable DC biasvoltage may be determined for all transducers of the batch.

By using a programmable DC bias-voltage generator, the production yieldof the MEMS microphones may be increased, while at the same time thesensitivity of individual microphones may be maximized.

As an example, the DC bias-voltage of the MEMS transducer element couldbe controlled to be between 5 and 10 Volts, depending on the stiffnessof the diaphragm. Furthermore, the DC bias-voltage could be maximizedwithout risking diaphragm collapse during normal operating conditions.This will result in better sensitivity and lower noise of the MEMSmicrophone assembly. Typically, the DC bias-voltage would not be changedcontinuously, but only adapted once in while or even just once, i.e.during production of the microphone.

By combining the adjustment of the gain and the bias voltage, it ispossible to compensate for both microphone and integrated circuitproduction variations by using internal calibration means to obtain abetter and more uniform microphone product, e.g. with less variation inelectroacoustical sensitivity and/or better signal-to-noise ratio.

Furthermore, the calibratable DC bias voltage and (pre)amplifier gaincalibration gives the production manager adjustable parameters that canbe used during full-scale production to improve the yield.

In a further embodiment, the processor may also be adapted to adjust oneor more further electrical parameters on the basis of the information inthe memory. Such parameter(s) may be a parameter of the signal from thediaphragm/MEMS transducer element, the output signal, or otherelectrical parameters of the microphone, such as parameters relating tothe internal operation of the microphone.

The present adaptation of such a parameter may be any adaptation of theparameter, such as on the basis of an internal or external power supplyor the changing of electrical components, such as the adding, removal orchanging of internal resistances, capacitances, impedances, inductancesor the like.

The adaptation in accordance with the information may be performed inany manner. The information itself may describe the adaptation, or itmay describe a desired parameter, where after the adaptation itself isdetermined by the processor. The adaptation of a model describing theadaptation may be provided internally in the microphone or be providedfrom an external source.

Another option is to adjust e.g. the sensitivity of the microphoneassembly by changing values of the electronic components of ananalogue-to-digital converter circuitry such as sampling and/or feedbackcapacitors. The programming of the calibration data can also be done inthe final test stage as described further below.

Re-programming may be an interesting option, which may require an extrasystem connection for entering an erase signal to the processor.Re-programming (after erase) may be triggered by applying again a‘write-level’ pulse to the programming pulse connection. Re-programmingmay be used for in-situ calibration of the system but it may require asound reference signal again.

In general, the communication with the memory and/or processor mountedinside the microphone housing, preferably on a suitable carrier such asa printed circuit board or ceramic substrate, may be obtained using anydesired known or new data communication interface and protocol, such asI2C or I2S. A preferred embodiment of the invention comprises alow-power, synchronous, bidirectional serial communication bus asdescribed in US Publication No. 2004/0116151 A1 or alternatively therelated SLIMbUS™ promoted by the MIPI Alliance. The memory mayadvantageously comprise transducer identification information forexample in terms of manufacturer's model and type designation inalpha-numeric, or any other suitable coded, format like “Sonion 8002microphone”. Furthermore or alternatively, component manufacturingspecific information such a production lot or batch number,manufacturing date and place, unique product ID etc, may stored in thememory. The memory may additionally or instead comprise performanceinformation related to the mechanical design or electrical and/oracoustical performance parameters of the transducer such as thepreviously-mentioned amplifier gain setting information and DC biasvoltage setting information. This will allow an external processor, forexample a DSP or microprocessor of a portable communication device likemobile phones and hearing instruments, to read the MEMS microphoneidentification information through the data communication interface forexample in connection with booting or power-on procedures. The DSP ormicroprocessor will be able to check whether the MEMS microphone is ofan appropriate/compatible type. Once the identity or performanceinformation of the transducer has been read by the DSP ormicroprocessor, it may adapt its operation accordingly through suitableprogram and software routines. Furthermore, it will be readily apparentto the skilled person that other types of miniature electro-acoustic ormagnetic transducers, such as moving coil and moving armature speakersand receivers, hearing aid telecoils etc, will be able to harvestcorresponding benefits from the integration of a memory that storestransducer identification and/or transducer manufacturing specificinformation. A set of particularly advantageous transducer embodimentscomprises a surface mountable transducer housing wherein all externallyaccessible soldering or connection terminals are arranged on asubstantially plane exterior surface of the transducer housing. For asurface mountable transducer, it will be practical to accommodate alarger number of externally accessible soldering or connection terminalssuch as 4-8 terminals because no manual soldering operations arerequired. The MEMS microphone according to the invention may have thefollowing advantages during production of analogue or digital condensermicrophones:

-   -   reduce the impact of semiconductor process variations from MEMS        and ASIC wafer production so as to minimize product parameter        variation of the final MEMS microphone product    -   enable higher tolerances on MEMS wafers    -   enable higher tolerance on ASIC bias generator level    -   enable higher tolerance on ASIC pre-amplifier gain    -   maximize the uniformity in microphone sensitivity    -   reduce the variation of the final product    -   maximize the production yield    -   minimize the MEMS and ASIC area.

Another aspect of the invention relates to a method of calibrating aMEMS microphone assembly. The method comprises the steps of (i)measuring or estimating a collapse voltage of the MEMS transducerelement, (ii) determining a DC bias voltage for the MEMS transducerelement on the basis of the measured or estimated collapse voltage, and(iii) writing information relating to the determined DC bias voltage tothe memory.

Naturally, the collapse voltage may be estimated or determined in anumber of manners. One manner is to gradually increase a DC voltagebetween the back plate and diaphragm of a single MEMS transducer anddetermine the collapse voltage of this MEMS transducer as the DC voltageat which the back plate and diaphragm actually touch or stick. Anothermethodology involves performing the same procedure on a test structure,representative of the MEMS transducer to make an indirect determinationof the collapse voltage of one or more MEMS transducers on the wafer.Preferably, the procedure is performed on a subset of MEMS transducerson a common wafer, such as 5-100 MEMS transducers, where the collapsevoltage of each MEMS transducer of the subset is determined. Thereafter,a single representative collapse voltage, such as an average or mean orweighted value, is derived from the values determined from the subset.

Another manner of determining the collapse voltage is one, whereinmeasuring/estimating step comprises the steps of, (i) applying a DC biasvoltage to the MEMS transducer element, (ii) applying a predeterminedsound pressure to the MEMS transducer element, (iii) measuring anacoustic sensitivity of the MEMS transducer element during theapplication of the DC bias voltage and the predetermined sound pressure,and (iv) determining the collapse voltage based on the measuredsensitivity and the applied DC bias voltage. The acoustic sensitivity ofthe MEMS transducer element depends on its diaphragm tension, which inturn relates to the collapse voltage. A look-up table can be createdbased on experimentally collected data for the relation between acousticsensitivity and collapse voltage of the MEMS transducer element for apredetermined DC bias voltage.

Yet another manner of determining the collapse voltage is one, whereinthe measuring/estimating step comprises the steps of (i) increasing a DCvoltage provided between the back plate and the diaphragm of the MEMStransducer element while monitoring a capacitance value between the backplate and the diaphragm, until, at a first voltage, a predeterminedincrease in the capacitance value is detected, and then (ii) estimatingthe collapse voltage on the basis of the first voltage.

When increasing the DC voltage, the distance or air gap height betweenthe back plate and the diaphragm will decrease, whereby the capacitancethere between will increase. This increase of transducer capacitance isnot linearly dependent on the air gap height and when an increased slopeof the capacitance versus DC voltage graph is seen, the collapse voltageis close. Thus, the collapse voltage may be determined or estimated froma voltage at which the slope has exceeded a predetermined slope or is apredetermined slope.

In general, the DC bias voltage may be determined according to a numberof different methods. The DC bias voltage may be determined as apredetermined percentage of the collapse voltage or the collapse voltagesubtracted a predetermined voltage. Other manners, of which one isdescribed further below, are also possible.

Naturally, collapse of the MEMS transducer element should be avoidedduring normal operation of the MEMS microphone assembly, even whensubjected to the specified maximum allowable sound pressure. Therefore,the DC bias voltage may be determined on the basis of the collapsevoltage subtracted a DC voltage corresponding to the peak AC voltagegenerated by the MEMS transducer element when subjected to the specifiedmaximum allowable sound pressure.

This is due to the fact that the distances traveled by the diaphragmduring sensing of a signal may be simulated by the providing of avoltage between the back plate and the diaphragm. As it is typicallydesired that the microphone is able to correctly measure sound pressuresup to a given maximum, this movement should be possible in spite of anyDC bias voltage applied. Thus, the voltage simulating this movement(such as that caused by a sound signal/pressure of 120-130 dB) isdetermined or estimated and is subtracted from the collapse voltage.

Subsequently to that, a further voltage, such as a safety marginvoltage, may be subtracted from the resulting voltage (collapse voltagesubtracted the voltage corresponding to the predetermined soundpressure).

In general, the method of the second aspect may further comprise thesteps of (i) applying a voltage corresponding to the determined DC biasvoltage to the MEMS transducer element, (ii) applying a predeterminedsound pressure to the MEMS transducer element, (iii) amplifying, in anamplifier, a signal output of the MEMS transducer element in response tothe sound pressure, and outputting an amplified signal, (iv)determining, on the basis of the amplified signal and a predeterminedsignal parameter, an amplifier gain setting, and (v) writing informationrelating to the determined amplifier gain setting to the memory.

Thus, as is also described further above, not only is the sensitivity ofthe actual MEMS transducer calibrated, but also that of the amplifiedsignal output of the assembly.

Preferably, this method further comprises the step of electricallyinterconnecting the MEMS transducer element and the amplifierpermanently on a common substrate carrier before performing the step ofdetermining the amplifier gain setting. In this manner, no changes willoccur in this interconnection subsequent to the calibration, which wouldotherwise reduce the accuracy of the calibration. Alternatively, theMEMS transducer element, the amplifier and optionally the memory and DCbias voltage generator may be integrated in a single semiconductor die.This will allow direct execution of the steps of determining theamplifier gain setting and writing the corresponding information to thememory without an intervening assembly step. For both methods, there isa considerable advantage in performing the amplifier gain setting on theassembled MEMS microphone assembly because the acoustical influence ofthe housing and the electrical influence of interconnections andimpedances are taking appropriately into account.

The step of determining the collapse voltage may advantageously beperformed on wafer level of the MEMS transducers, which allows directaccess to the back plate and diaphragm structures for the application ofthe DC voltage from a wafer tester. Alternatively, the controllable biasvoltage generator, normally used for providing the DC bias voltage ofthe MEMS microphone assembly, could be utilized in the step ofdetermining the collapse voltage. This could be obtained through acycle, wherein the MEMS microphone assembly is re-programmed through anumber of steps to gradually increase the DC bias voltage across thediaphragm and back plate.

Thus, the step of measuring or estimating a collapse voltage of the MEMStransducer element is preferably performed on a MEMS transducer wafercomprising a plurality of MEMS transducers.

Also, the collapse voltage of the MEMS transducer element may beestimated from a MEMS microphone subset of the plurality of MEMSmicrophones.

A third aspect of the invention relates to a method of calibrating aplurality of MEMS microphone assemblies. The method comprises (i)providing a plurality of MEMS transducer elements from a single batch ora single wafer, (ii) providing a MEMS transducer element in eachmicrophone assembly, (iii) calibrating a subset of the plurality of MEMSmicrophone assemblies in accordance with the second aspect of theinvention and obtaining DC bias voltage information there from, and (iv)writing at least the obtained DC bias voltage information to respectivememories of the remaining microphone assemblies of the plurality of MEMSmicrophone assemblies.

Consequently, it is assumed that the production parameters and theparameters of the MEMS transducers vary sufficiently little over thebatch or wafer, where a batch may comprise 1, 2, 3, 4, or more, such as12 or more wafers, so that the DC bias voltage determined from thesubset may be applied to all of the assemblies.

Naturally, the method may subsequently comprise also calibrating thegain of an amplifier of each assembly as described in relation to thesecond aspect. This calibration may be a separate calibration of eachassembly or a calibration derived again from a subset of the assemblies(the same or another subset), and the calibration data derived therefrommay then also be transferred to the memories of all assemblies.

If the calibration of the DC bias voltage and/or the amplifier gain isperformed on a subset of two or more assemblies, the resultingvoltage/gain may be derived from those obtained from the calibration inany manner, such as deriving a mean value of those obtained from thecalibration, a weighted mean value, or where obviously erroneous results(either from the measurement or stemming from malfunctioning assemblies)are discarded.

It may also, during the calibration, be determined whether the variationover the initial batch/wafer is sufficiently small for all assemblies tobe covered by the same calibration. If not, the batch/wafer may bedivided into smaller batches/parts of the wafer, inside which thecalibrations may be transferred to other assemblies. Thus, thecalibration may be only between assemblies stemming from parts of awafer or only some wafers of a batch, where other parts/wafers arecalibrated on the basis of assemblies (or rather transducers/amplifiers)produced at that area/wafer.

In general, it should be noted that in the present specification andclaims, the term “microphone housing” is to be construed broadly. In oneembodiment of the invention, the microphone housing comprises anelectrically conductive lid mounted to a substrate carrier in anacoustically sealed manner. The MEMS transducer element is attached tothe substrate carrier and electrically connected to substrate conductorsby flip-chip mounting or wire bonding. The sound inlet may be positionedin the lid or the substrate carrier or both of these to form adirectional microphone assembly. In another embodiment of the invention,the microphone housing is formed by outer surfaces of the MEMStransducer element, the substrate carrier, and optionally an ASIC die,that are bonded together to form a ultra compact so-called chip scalepackage (CSP) wherein the housing is an integral part of the MEMStransducer element and the substrate carrier.

BRIEF DESCRIPTION OF THE INVENTION

In the following, a preferred embodiment of the invention will bedescribed with reference to the drawing, wherein:

FIG. 1 illustrates a general diagram of important elements of thepreferred embodiment of a microphone of the invention, and

FIG. 2 illustrates a manner of determining a bias voltage.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of a microphone 10 of the invention comprises aMEMS condenser microphone/transducer 12 with an integrated circuitportion 14 which comprises a microphone (pre)amplifier 16, a DC biasvoltage generator 18 and is built into a microphone housing/package 20.In addition, the microphone has a voltage supply 111 and an output 15.

The amplifier 16 comprises an input for data 22 for adjusting the gainthereof, and the bias voltage generator 18 comprises a diode set-up 26and a Dickson pump 24 (see e.g. EP-A-1 599 067 which is hereinincorporated by reference in its entirety) having an input for data 28for regulating the voltage output of the generator 18. The operation ofthe Dickson pump 24 is a direct conversion of the information of the Mbits to a voltage.

The gain of the microphone preamplifier 16 is adjusted by the use ofcalibration data 22 that are loaded into and stored in a portion of anon-volatile memory 30 of the integrated circuit 14 during a final teststep in the production process of the MEMS condenser microphone 10.Additionally, the data for use in the generator 18 are stored in anotherportion of the memory 30.

Preferably, the non-volatile memory 30 comprises One-Time-Programmable(OTP) memory such as EPROM, fuse-based memory or similar types ofelectronic memory. However, multi-programmable memory types such asEEPROM and/or Flash memory may be utilized in other embodiments of theinvention, especially if these types of memory devices are already inuse for other purposes on the integrated circuit.

The programming process of the MEMS condenser microphone 10 may inpractice proceed along the below-mentioned steps:

A well defined sound pressure of predetermined level (e.g. 94 dB SPL/1kHz sine-wave) is applied to the individually packaged microphone 10while the electrical output signal of the MEMS condenser microphonetransducer 12 is measured. The MEMS condenser microphone 10 mayadvantageously be located in a suitable test jig inside an acousticaltest box.

In the preferred embodiment according to FIG. 1, the gain of themicrophone preamplifier 16 is adjusted or calibrated by varying theratio of either a set of resistors or a set of capacitors thereof thatare coupled as a feedback network of a microphone preamplifierconfiguration. The feedback microphone preamplifier 16 can be eithersingle-ended or differential.

The sensitivity of the MEMS transducer assembly is adjusted by adjustingthe value of the DC bias voltage (see below in relation to FIG. 2).

In the present embodiment, the sensitivity of the MEMS transducerassembly 10 is measured, recorded and tracked in a test computer to thestage of final microphone assembly and test where the presentcalibration process is carried out. Based on the known sensitivity ofthe MEMS transducer assembly 10, an appropriate value for the DC biasvoltage is determined/calculated by the test computer and thereafterprogrammed into the OTP memory 30 by choosing the appropriate code forexample through a pre-stored lookup table.

FIG. 2 illustrates a particularly useful manner of estimating ordetermining a desired bias voltage for the MEMS transducer 12. A varyingvoltage is provided between the back plate and diaphragm of the MEMStransducer 12, whereby the air gap height (the distance between thediaphragm and back plate) will vary. This height may be estimated on thebasis of a capacitance built between these elements. This capacitancevalue, however, is not linear with the distance but will increasedrastically when the distance is close to zero. Zero distance is aso-called “collapse” where the diaphragm touches the back plate.

FIG. 2 illustrates the capacitance C as a function of a voltage Vapplied between the diaphragm and back plate. It is seen that Cincreases drastically, when V is close to the collapse voltage,Vcollapse, which is the lowest voltage required for having the backplate and diaphragm touch.

Thus, from this graph, Vcollapse may be estimated even without bringingthe voltage V applied between the back plate and diaphragm to Vcollapse.

However, using a bias voltage close to Vcollapse will not provide thedesired sensitivity of the microphone 10 in that once a sound pressureacts on the diaphragm, this will force the diaphragm toward the backplate and may cause collapse. Thus, the theoretically largest biasvoltage should be Vcollapse subtracted a voltage which corresponds tothe largest variation of the diaphragm-back plate distance caused by thelargest sound pressure (or other phenomenon, such as acceleration causedby the microphone being dropped) which the microphone should be able tosense. This variation is illustrated by a varying curve illustrating avoltage variation required to simulate the variation caused by thesound, which may be, for example, 120-130 dB.

Thus, half this Vp-p should be subtracted from Vcollapse, and preferablya margin voltage, Vmargin, is also subtracted in order to ensure thatcollapse is not encountered during normal or expected operation.

As a result of this analysis, Vbias may be determined as Vcollapsesubtracted Vmargin and half of Vp-p.

Once the OTP memory 30 has been programmed with the appropriate code,the test process is preferably halted for a short moment to allow themicrophone output signal to settle to its correct bias point after theprogramming of the DC bias voltage.

Thereafter, the MEMS condenser microphone sensitivity is measured andthe target and appropriate preamplifier gain is calculated on the basisof the measured sensitivity and a pre-stored reference sensitivity.Finally, from the target preamplifier gain, an appropriate code isdetermined and programmed into the corresponding OTP memory area.Optionally, a final calibration procedure step may be executed thatcomprises re-measuring the sensitivity of the MEMS condenser microphoneto confirm that the actual measured value is within the expectedsensitivity range that may have a band of +/−1 or 2 dB around thenominal sensitivity value.

The programming of the non-volatile memory 30 can be done with a verysimple serial data interface 32 that may comprise a clock and a datasignal or single signal line with composite data/clock signals that areaccessible on respective external programming pin(s) of the microphoneassembly 10. A state machine inside the integrated circuitry 14 isadapted to decode the incoming data stream and handle the writing ofmemory data to the OTP memory 30.

In the case of a digital microphone assembly, the external programmingpin(s) 32 may be shared with already provided digital input/output pinssuch as Left/Right signal or other digital signals. For MEMS microphones10 that are packaged in a SMD mountable package, the extra space andsolder connections required by additional external programming pin(s) isa minor concern.

For an analogue microphone assembly it will normally be necessary to addthe external programming pin(s) 32 to the already existing externalpins. This addition can however be done at substantially no extra cost.

While the present invention has been described with reference to one ormore particular embodiments, those skilled in the art will recognizethat many changes may be made thereto without departing from the spiritand scope of the present invention.

1. A MEMS microphone assembly including a microphone housing, the MEMSmicrophone assembly comprising: a sound inlet; a MEMS transducer elementhaving a back plate and a diaphragm displaceable in relation to the backplate; a controllable bias voltage generator adapted to provide a DCbias voltage between the diaphragm and the back plate; a memory forstoring information including amplifier gain setting information; acontrollable amplifier for receiving an electrical signal from the MEMStransducer element and providing an output signal, the controllableamplifier being adapted to amplify the electrical signal from the MEMStransducer in accordance with an amplifier gain setting; and a processoradapted to retrieve information from the memory and to control the gainof the amplifier in accordance with the amplifier gain settinginformation from the memory, and control the bias voltage generator toprovide a DC bias voltage in accordance with the information from thememory.
 2. A MEMS microphone assembly according to claim 1, wherein theMEMS transducer element has a distance from the back plate to thediaphragm in the range of 1-10 μm, such as 2-5 μm.
 3. A MEMS microphoneassembly according to claim 1, wherein the controllable bias voltagegenerator is adapted to generate a DC bias voltage in the range of 5-20V.
 4. A MEMS microphone assembly according to claim 1, wherein thememory comprises memory circuitry of a type of the group consisting of:RAM, PROM, EPROM, EEPROM, flash, one-time-programmable memories, andmemories based on fuse-link technology.
 5. A method of calibrating aMEMS microphone assembly comprising a MEMS transducer element, themethod comprising the steps of: measuring or estimating a collapsevoltage of the MEMS transducer element; determining a DC bias voltagefor the MEMS transducer element on the basis of the measured orestimated collapse voltage; and writing information relating to thedetermined DC bias voltage to a memory of the microphone assembly.
 6. Amethod according to claim 5, wherein measuring/estimating step comprisesthe steps of: applying a DC bias voltage to the MEMS transducer element,applying a predetermined sound pressure to the MEMS transducer element,measuring an acoustic sensitivity of the MEMS transducer element duringthe application of the DC bias voltage and the predetermined soundpressure, and determining the collapse voltage based on the measuredsensitivity and the applied DC bias voltage.
 7. A method according toclaim 5, wherein the measuring/estimating step comprises the steps of:increasing a voltage provided between the back plate and the diaphragmof the MEMS transducer element while monitoring a capacitance valuebetween the back plate and the diaphragm, until, at a first voltage, apredetermined increase in the capacitance value is detected, andestimating the collapse voltage on the basis of the first voltage.
 8. Amethod according to claim 5, further comprising the steps of: applying aDC voltage corresponding to the determined DC bias voltage to the MEMStransducer element, applying a predetermined sound pressure to the MEMStransducer element, amplifying, in an amplifier, a signal output of theMEMS transducer element in response to the sound pressure, andoutputting an amplified signal, determining, on the basis of theamplified signal and a predetermined signal parameter, an amplifier gainsetting, and writing information relating to the determined amplifiergain setting to the memory.
 9. A method according to claim 8, furthercomprising the step of electrically interconnecting the MEMS transducerelement and the amplifier permanently on a common substrate carrierbefore performing the step of determining the amplifier gain setting.10. A method according to claim 5, wherein the step of measuring orestimating a collapse voltage of the MEMS transducer element isperformed on a MEMS microphone wafer comprising a plurality of MEMSmicrophones.
 11. A method according to claim 10, wherein the collapsevoltage of the MEMS transducer element is estimated from a MEMStransducer subset of the plurality of MEMS transducers.
 12. A method ofcalibrating a plurality of MEMS microphone assemblies, the methodcomprising: providing a plurality of MEMS transducer elements from asingle wafer batch or a single wafer, providing a MEMS transducerelement in each microphone assembly; calibrating a subset of theplurality of MEMS microphone assemblies in accordance with the method ofclaim 5 and deriving DC bias voltage information there from; and writingat least the derived DC bias voltage information to respective memoriesof the remaining MEMS microphone assemblies of the plurality of MEMSmicrophone assemblies.